The present invention relates to systems for controlling the application of power to alternating current induction motors; and in particular to such systems which incorporate a mechanism for measuring the rms value of the current flowing through the motor.
One type of a three-phase induction motor has three wye connected stator windings. A conventional controller for this type of motor has separate thyristor switches connecting a stator winding to one of three alternating current supply lines. Each thyristor switch is formed by either a triac or a pair of inversely connected silicon controller rectifiers (SCR's). A circuit within the controller determines the proper time at which to trigger each thyristor switch during every half-cycle of the associated supply line voltage. A triggered thyristor switch remains in a conductive state until the alternating current flowing through it goes to zero, at which time the thyristor must be triggered to become conductive again. By altering the trigger times of the switches with respect to the zero crossings of the supply line voltage (or with respect to the current cessation time), the intervals during which they are conductive can be varied to control the amount of voltage applied to the motor.
To start the motor, conventional motor controllers vary the thyristor switch trigger times to provide a gradual increase in the voltage. In doing so, the switches are initially triggered relatively late in the voltage half-cycles and are conductive for only a short period. The trigger times then become progressively earlier in each half-cycle to render the thyristor switches conductive for longer intervals and apply greater amounts of voltage to the motor until it reaches full speed. An inverse technique can be used to reduce the motor speed.
Although measuring the rms level of the current flowing through the motor is not required for the operation of the basic motor controller, such measurement often is desirable as a indicator of the malfunction of equipment being driven by the motor. For example, relative changes in the current may reflect a problem along an assembly line and the need to take corrective measures. The current level also can be used to detect a current overload and the need to shut off the motor until the condition that produced the overload is corrected. Heretofore, a current sensing transformer was typically placed in the supply line to produce a signal in its secondary coil that corresponded to the motor current. This signal was measured to provide an indication of the motor current magnitude.